I think the concensus is to use the JG tubing, (the black is UV resistant) I haven't seen any 1/4 PEX but I think someone found it online somewhere
Pre-charge the accumulator 2 lbs below the cut-in pressure you have your diferential pressure switch set for, if the 100lbs is that cut in pressure then you would precharge to 98lbs.
If they are rated to take the pressure you can use them but the JG fittings are very reliable so you are less likely to have problems if you stick with them
Are you planning on using the buckets or some other type of chamber, a larger chamber should allow you to use fewer nozzles. I think you would probably need at least two solenoids with 36 misters but it depends on the complete setup.
Yes, I don't know the math behind the fluid dynamics but the people who do say it helps
The Aquatec pumps are primarily used in water filtering applications and those usually use 1/4" tubing, so they usually stock that size. We really aren't the target market they are going for